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Sorghum Insects

Chinch Bug, Blissus leucopterus.

Adult chinch bug.

Adult with nymphs.

Aggregation of chinch bugs on sorghum plant.

Rows of sorghum killed by chinch bugs.

Identification

Adults are small, black bugs about 1/8 inch long with white wings folded over the back. Two small, dark, triangular markings appear near the mid-portion of the wings. Immatures are bright red after hatching, then darken as they approach maturity. A white band on the upper side of the first abdominal segment is visible until the wing buds grow to cover it.

Biology

Overwintered adults emerge in early spring and fly to small grains where they mate and produce the first generation. Most problems in sorghum occur when large groups of immature, wingless nymphs migrate from fields as the grain matures and invade adjacent sorghum fields to mature and produce a second generation. Because the nymphs are flightless, various barrier treatments and trap crops historically have been used to protect the margins of emerging sorghum fields. Occasionally, adults fly into sorghum either directly from overwintering grasses, late-maturing wheat or from other sorghum fields.

Problems with this insect are generally confined to eastern and central Kansas, with damage beginning in May or June. The risk of damage is greater where sorghum is planted next to thin stands of wheat. Seedling sorghum is most vulnerable, and seven to 10 bugs per plant will cause stunting, poor root development and stand reduction. Larger plants can tolerate more bugs, but severe infestations can cause stunting, lodging and yield loss. Chinch bugs usually increase in dry periods and decline during wetter years. Outbreaks tend to occur in roughly 7- to 10-year cycles. Locally damaging infestations develop nearly every year, particularly in parts of south central Kansas. Growers should continue to monitor for chinch bug activity each spring, especially when moisture is limited during April, May and June and nearby wheat is thin and lacking in vigor.

Chinch bug management

2. Consider planting a trap crop of sorghum or sudangrass as early as possible in a 50-foot barrier strip between wheat and sorghum fields. If migration appears heavy, the barrier strip can be sprayed. Note: Soybean is not a preferred food plant and will not function as a trap crop. Migrating bugs may move up to a quarter-mile or more on foot through the soybeans in search of a suitable host.

3. Choose hybrids carefully. No sorghums have strong resistance, but some tolerate damage or respond to treatment better than others.

5. Scout small grain fields in May and June. Infestations of one adult or four to five nymphs per row foot indicate impending damage to border rows of adjacent sorghum.

6. Where problems are anticipated, consider using planting-time insecticides. Sorghum planted within about three weeks of small grain maturity benefits the most. The optional approach of relying entirely on foliar sprays during the seedling period can prove unsatisfactory. In one study, a delay of only two days in spraying 3-inch sorghums resulted in an 80 to 90 percent stand loss where moderate numbers of chinch bugs were present.

7. Remain vigilant. The protection afforded by seed treatment does not extend much beyond two weeks. Growers should stay vigilant throughout wheat maturity and prepare to apply follow-up sprays on border rows if chinch bugs reach five or six per seedling plant. Larger plants can tolerate more bugs, but watch for differences in plant growth and root development between infested and noninfested areas as indications that spot treatments are justified. Plants one foot high to flowering can be damaged by 50 or more bugs per plant.

8. Integrate tactics. Optimal chinch bug management is best achieved with an integrated approach. This means combining careful hybrid selection with cultural controls, seed treatment, monitoring and rescue treatments rather than relying on a single control tactic.

Seed treatments

Using seed treated with imidacloprid or thiamethoxam at planting can decrease damage. These treatments work well initially, but wear off in a few weeks. Growers should be aware of this problem and use follow-up sprays on border rows if the early season protection wears off before the end of chinch bug migration.